Abstract: :
Purpose: The alpha-dicarbonyl compounds, methylglyoxal (MG)and glyoxal react with proteins and produce protein-proteincrosslinking structures and fluorescent products. We have previouslyshown the presence of imidazolium protein crosslinking structuresand a fluorescent product, argpyrimidine in human lens proteins.Our studies have also shown that these products accumulate inrelatively large quantities in highly pigmented cataractouslenses (Chellan and Nagaraj, ABB 368: 98, 1999; Padayatti etal., IOVS 42:1299, 2001). In this study, we have determinedthe effect of pyridoxamine (PM) on the formation of alpha-dicarbonyl-mediatedmodifications in lens proteins of diabetic rats and studiedeffects on enzymes that can metabolize alpha-dicarbonyl compounds.Methods: Four groups of Sprague-Dawley male rats (12 in eachgroup, 150 grams) were used. Diabetes was induced in two groupsof rats by injecting streptozocin. PM was administered throughdrinking water at 400 mg/L in diabetic rats and at 800 mg/Lin non-diabetic control rats. After five months of treatment,imidazolium crosslinks, GOLD (glyoxal-lysine dimer) and MOLD(methylglyoxal-lysine dimer) and, argpyrimidine and pentosidinewere measured in lens proteins by HPLC. Activity of two alpha-dicarbonylmetabolizing enzymes, aldose reductase and glyoxalase I wasmeasured. Glutathione (GSH) was measured in the protein-freeextracts of the lens. Results: The GOLD levels were ∼7-fold higherin diabetic rats when compared to non-diabetic controls. PMtreatment reduced these levels by nearly 50% in diabetic rats.MOLD levels were ∼2-fold higher in diabetic rats and were almostnormalized by the PM treatment. Argpyrimidine and pentosidinelevels were higher in diabetic rats and were corrected by thePM treatment. Surprisingly, the pentosidine and argpyrimidinelevels were below normal in PM treated non-diabetic rats. GlyoxalaseI activity was significantly reduced in diabetes, but this wasprevented by PM treatment. Aldose reductase activity was increased2.3 times in diabetes and PM treatment further increased thisactivity by 25%. Diabetes lowered GSH levels by 65%, PM hadno effect on GSH levels. Conclusion: PM is able to block alpha-dicarbonylmediated modification of lens proteins. This could be due toinhibition of the reaction of alpha-dicarbonyls with lens proteinsand/or activation/retention of enzymes that can metabolize alpha-dicarbonylcompounds.